The present invention relates to boundary-layer-ingesting inlets for aircraft engines and more specifically a system for reducing distortion at the aerodynamic interface plane using a combination of active and passive flow control devices.
The effect of aviation on the environment and in particular global warming has recently become a focus of study. This study considers the three primary impacts that civil aviation has on the environment. These impacts include aircraft noise and emissions pollution around airports and emissions at altitude. In response to environmental concerns and to foster revolutionary propulsion technologies the feasibility of the Blended-Wing-Body (BWB) concept as an efficient alternative to conventional transport configurations was explored. The BWB concept has been considered in various forms for several years. System studies have shown that in order to make the largest impact on the vehicle performance, the engines and inlets should be placed near the upper surface on the aft section of the vehicle. However, the incorporation of the inlets on the surface of the vehicle increases the technical risk of the configuration. While preliminary designs have avoided this risk by positioning the engines on pods the use of boundary-layer-ingesting (BLI) flush-mounted, offset inlets, will increase our understanding of the technical risk and benefits of this type of inlet design and how to best mitigate them using flow control. Additional system studies continue to indicate the advantages of BLI inlets for the BWB configuration including less fuel burn and lower noise characteristics.
When the engines are positioned near the surface, the BWB engine inlet will likely be an S-shaped duct with the capability to ingest the large boundary layer generated over the aircraft body. The inlet must perform this task while also meeting standard inlet distortion and pressure recovery performance requirements. Since the boundary layer on the BWB is expected to be on the order of about 30% of the inlet height, this presents a challenging task for inlet design. In addition, the performance assessment of such a highly-integrated propulsion system is a complex undertaking, requiring the simultaneous examination of many influential factors in order to determine whether BLI provides a benefit from a system standpoint. The trade-offs among reduced drag, weight savings or penalty, and engine operation must all be considered to assess the relative benefit of BLI technology. However, in order for the engine to operate in the BLI environment, a minimum distortion level must be achieved even at the cost of reduced efficiency. The effect of BLI on engine performance is known to be detrimental because BLI increases the distortion and reduces the pressure recovery at the engine fan-face. Despite the loss in engine performance, the benefit of BLI must be addressed from an overall systems level viewpoint.
This requirement for at least a minimum level of inlet performance under the severe conditions of an S-duct and a very large onset boundary layer flow have led to the consideration of flow control devices in the inlet to control the flow in this type of configuration. Passive flow control in the form of vortex generating (MG) vanes can be used to improve the inlet flow. A drawback of vortex generating vanes or other forms of passive flow control devices is that they are generally optimized around a small envelope and thus become less effective when the inlet conditions are outside of that envelope. Other drawbacks for passive flow control include increased losses of the inlet flow since passive devices extract energy from the flow for control.
Additionally, active flow control methods, such as active flow control jets, have also been investigated as a means to improve inlet flow for aggressive serpentine inlets with minimal BLI. Active flow control devices have an advantage over passive flow control devices in that the strength of the actuators can be varied as needed. Unlike passive flow control devices, active flow control devices can be adjusted to minimize the inlet distortion over the entire operation of the inlet. Known drawbacks of active flow control include the losses to engine efficiency due to the amount of inlet mass flow required to drive the flow control and the added complexity of plumbing and operating the active flow control.
One type of active flow control device is disclosed in U.S. Pat. No. 6,371,414, issued to Truax et al (Truax). In Truax, flow behavior of a ducted flow is controlled using very-small-scale effectors. These effectors are based on micro-fabricated-electro-mechanical system (MEMS) technology to sense flow conditions and activate the very-small-scale effectors. The very-small-scale-effectors can be fluidic effectors for creating; a secondary flow, pulsing effectors, synthetic flow effectors, micro-bubble effectors. The very-small-scale effectors are used to induce and manipulate vortex formation to control the lateral layer separation within the fluid flow.
However, there remains a need for additional methods and apparatus to improve BLI inlet flow across the range of flow conditions while minimizing the negative impacts of active flow control.